1. Field of the Invention
The present invention relates to a composite integrated circuit and its fabrication method, and more particularly to an oxide composite integrated circuit and its fabrication method enabling oxide thin films to be put to practical use as electronic devices.
2. Description of the Related Art
Generally, oxide thin films have a wide variety of physical properties such as dielectric, magnetic, superconducting properties, and are applicable to various electronic devices. To derive from these oxide thin films the characteristics equivalent to those of bulk crystals, it is essential for them to be epitaxially grown on a well controlled crystalline substrate. Conventional oxide substrates, however, are costly, and are unsuitable to produce on a large scale.
As mobile devices have been developed recently, further progress has been made in miniaturization of electronic circuits. In conjunction with this, it becomes more important to miniaturize capacitors as essential components for various circuits. At present, most electronic circuits are based on silicon, and hence when considering the miniaturization and monolithic devices such as thin film capacitors, it is necessary to merge them with the conventional silicon process.
Thus, trials to epitaxially grow oxide thin films on silicon substrates have been made intensively. In particular, as for the gate oxide films as a substitute for SiO2, various trials have been made such as Ta2O5 as high-k material. However, as is widely known, since silicon substrate surfaces have very high activity, it is extremely difficult to grow oxide thin films on interfaces without SiO2.
Up to now, using (111) silicon substrate, Yoshimoto et al. of Tokyo Institute of Technology have succeeded in the film deposition of (111) CeO2 without generating any SiO2 film. In addition, Mckee et al. in Oak Ridge National Institute have succeeded in immediate film deposition of SrTiO3 by suppressing the activity by forming SrSi on (100) silicon surface.
The foregoing reports, however, discloses only observations carried out on a lattice image level, and no reports have been made that the epitaxial growth was achieved successfully in an millimeter order or on the entire surface of a wafer. This is probably because the epitaxial growth on silicon must consider not only the lattice matching within a surface, but also the lattice matching in the direction of the film thickness.
FIG. 2 is a schematic diagram showing an example of the epitaxial growth when SrTiO3, one of the oxide thin films, is grown on a (100) silicon substrate with a single step. A one-molecular layer of the oxide thin film of STO is about 4 Å, but a single step level difference of the (100) silicon substrate is 1.38 Å. Thus, when the oxide thin film is grown on the substrate, the lattice will deviate every step, bringing about an anti-phase boundary 18, and probably resulting in a microcrystal. In FIG. 2, the reference numeral 16 designates a (100) silicon single step, and 17 designates a (100) SrTiO3 single step.
Since the current silicon process is optimized for the most stable (100) substrates, there is no freedom of substrate selection for the film deposition of the oxide thin films when applying them to monolithic devices. For example, when growing CeO2 on (100) silicon, a (110) CeO2 surface grows, causing a twin, resulting in a microcrystal.